If you're looking for a QuickBooks Product to manage your customers offline, then QuickBooks Desktop would be your best bet. While you need an internet connection to purchase and download the product from the internet. You can use it offline. However, it's important to note that the Desktop product is used for much more than just customer management, the number of tools at your disposal include income tracking, ability to create invoices, other reports, estimates, sales tax tracking and much more. You can view the different products and their feature here. I also highly suggest reaching out to the sales team at the phone number in the link provided, they know all the ins-and-outs of all Intuit products and would be able to find the perfect one to fit you and your business model like a glove.
If you have any other questions, feel free to post here, thank you for your time and have a lovely evening.